Trying to get sizzle sounds from Cym. Line all but the default to a sizzle/suck sound no matter what notehead number I use.  Trying to use 51 for a sizzle sound with a tenuto mark.  All of the other sounds work fine, but for some reason I can't get these (all of the sizzle sounds) to work.

Windows 10
Sibelius 8.5
VDL 2.5.5
Kontakt 5

I'm not sure. It sounds like you're doing it correctly, and perhaps there's something in your working file that's causing the issue.

One way to maybe "hack" it would be to enter a tie on the sizz note (still with a tenuto and notehead 51), then hide the tie (ctrl.-shift-H) if you want. Doing this, you're essentially bypassing the note-off message which is when the "suck" release is being triggered.